1. Field of the Invention
The present invention relates to a semiconductor light emitting device and its manufacturing method, said light emitting device being used as a semiconductor laser device, as a blue light emitter which is one of the elements of a display panel for use in displays of various electronic apparatuses, as a blue light emitting device (LED) used individually in a display apparatus, as a signal reading and writing light emitting device for use in a compact disk (CD) player, a laser disk (LD) player and a magnetic optical disk player, and as a light emitting device for use in a bar code reader.
2. Description of the Prior Art
FIG. 1 schematically shows the basic structure of a semiconductor laser device which is a kind of such a semiconductor light emitting device, and the condition of a corresponding energy band. The general structure of the semiconductor laser device is as follows: a semiconductor film B is formed on the surface of an N-type semiconductor substrate A by MBE (molecular beam epitaxy)-growing an N-type semiconductor layer B.sub.1, an active layer B.sub.2 and a P-type semiconductor layer B.sub.3 in this order, and light is emitted from the active layer B.sub.2 by applying a bias voltage in a forward direction between a metal electrode E.sub.1 formed on the reverse surface of the substrate A and a metal electrode E.sub.2 formed on the obverse surface of the P-type semiconductor layer B.sub.3 which is the top surface of the semiconductor film B, i.e. from the electrode E.sub.2 to the electrode E.sub.1.
As well known, the energy band of the semiconductor laser device of the above-described structure is of a configuration such that the energy levels of the N-type semiconductor layer B.sub.1 and the P-type semiconductor layer B.sub.3 are high and that an energy level trough is formed at the active layer B.sub.2 which is a P-N junction. An energy barrier .DELTA.V is generated between the electrodes E.sub.1 and E.sub.2 and the semiconductor film B.
Therefore, when a voltage necessary to obtain a current I which causes holes h to go over the energy barrier .DELTA.V is applied between the electrodes E.sub.1 and E.sub.2, the carriers i.e. holes h and electrons injected by the voltage application are shut up in the active layer B.sub.2 where the energy level is low, so that an induced emission occurs vigorously. When the exciting current exceeds a threshold value, light resonates between the parallel end surfaces of the active layer B.sub.2 to cause a laser oscillation.
FIG. 2 shows an example of a more specific structure of a conventional semiconductor laser device. The device shown in this figure is what is called a blue light emitting semiconductor layer of ZnSe in which an N-type GaAs substrate 21 is used as the N-type semiconductor substrate and a group II-VI semiconductor film 22 of ZnCdSSe or MgZnCdSSe is formed on the substrate 21 as the semiconductor film.
The group II-VI semiconductor film 22 is formed by MBE-growing an N-type ZnSe layer 23 which is a buffer layer, an N-type ZnSSe layer 24 which is a clad layer, a ZnCdSe layer 25 which is an active layer, a P-type ZnSSe layer 26 which is a clad layer and a P-type ZnSe layer 27 which is a buffer layer in this order on the substrate 21. A metal such as Au is directly deposited onto the P-type ZnSe layer 27 which is the top layer of the group II-VI semiconductor film to form a positive electrode 28. Reference numeral 29 represents a negative electrode formed on the reverse surface of the substrate 21.
In the semiconductor laser device of the above-described conventional structure, the metal electrode 28 is directly formed on the P-type ZnSe layer 27. However, it is known that when the P-type semiconductor of ZnSe is directly joined to a metal, a Schottky-type voltage/current characteristic exists therebetween.
Specifically, in the conventional structure, as is shown in the energy band configuration of FIG. 3, when a bias voltage is applied in a forward direction between the electrodes 28 and 29, a steep Schottky-type energy barrier .DELTA.V is generated between the metal electrode 28 and the P-type ZnSe layer 27 forming the outermost layer of the group II-VI semiconductor film 22. For this reason, a current which causes the holes h to go over the energy barrier .DELTA.V cannot be obtained unless a considerably high voltage is applied.
Therefore, in the conventional structure, since not only the power required to drive the device increases but also a current of several amperes flows in the device, the current density in the device is very high, so that it is inevitable for the device to generate heat while being driven. Thus, in the conventional device, in addition to the increase in power consumption, it is difficult to operate the device at ordinary temperature because of the possibility of thermal breakdown.
To solve these problems, it is necessary for the device to have a structure where currents flow easily from the metal electrode 28 to the group II-VI semiconductor film 22 so that the bias voltage applied between the electrodes 28 and 29 may be as low as possible. To do so, for example, after the electrode 28 is formed, the group II-VI semiconductor film 22 may be maintained at a temperature higher than its growth temperature.
Specifically, the MBE growth of the group II-VI semiconductor film 22 on the substrate 21 is performed normally at a substrate temperature of 350.degree. or below. After the metal electrode 28 is deposited onto the group II-VI semiconductor film 22, the semiconductor film 22 is heated again to a temperature (e.g. approximately 400.degree. C.) higher than the growth temperature so that the metal constituting the electrode 28 is diffused into the group II-VI semiconductor film 22.
Since the inclination of the energy barrier .DELTA.V is gentler as shown by the broken line of FIG. 3 when the metal is diffused into the P-type ZnSe layer 27 which is the top layer of the group II-VI semiconductor film 22, the current flows more easily. However, the group II-VI semiconductor film 22 has the property of increasing in electric resistance when heated to a temperature higher than the growth temperature.
For this reason, it is difficult under the present circumstances to perform the alloying by diffusing the electrode metal into the P-type ZnSe layer 27 while maintaining the electric resistance of the ZnSe layer 27 to be low. Therefore, to obtain the necessary current, a high voltage must be applied between the electrodes 28 and 29 like in the above-described conventional structure. Thus, the problems cannot be solved by this method.
As another solution, a Group II-VI semiconductor film 22 having a high carrier concentration of 10.sup.19 /cm.sup.3 or above may be Grown on the substrate 21. According to this method, since the energy band of the P-type ZnSe layer 27 shifts to decrease the energy barrier as shown by the alternate long and short dash line of FIG. 3, a structure in which the current flows easily is realized. However, under the present circumstances, it is technically next to impossible to obtain a P-type Group II-VI semiconductor film having such a high carrier concentration.